The present invention relates to methods for the treatment of wood pulp, and more particularly to methods for oxygen delignification of the brownstock produced during standard pulping operations. In particular, the present invention is directed to the production of wood pulp having greatly improved strength concomitant with high degrees of delignification.
Wood is comprised in major proportion of cellulose and hemicellulose fiber and amorphous, non-fibrous lignin which serves to hold the fibrous portions together. The hemicellulose and the cellulose fibers are sometimes referred to collectively as holocellulose. During the treatment of wood to produce pulp, the wood is transformed into a fibrous mass by removing a substantial portion of the lignin from the wood. Thus, processes for the production of paper and paper products generally include a pulping stage in which wood, usually in the form of wood chips, is reduced to a fibrous mass. Several different pulping methods are known in the art; they are generally classified as mechanical, chemical or semi-chemical pulping.
Although mechanical pulping is still used in the paper making industry, chemical and semichemical pulping currently account for about 70% of North American production. Chemical pulping methods include a wide variety of processes, such as the sulphite process, the bisulfite process, the soda process and the Kraft process. The Kraft process is the predominant form of chemical pulping.
Chemical pulping operations generally comprise introducing wood chips into a digesting vessel where they are cooked in a chemical liquor. In the Kraft process, the cooking liquor comprises a mixture of sodium hydroxide and sodium sulphide. After the required cooking period, softened and delignified wood chips are separated from the cooking liquor to produce a fibrous mass of pulp. The pulp produced by chemical pulping is sometimes called "brownstock" because it is typically too dark in color for the production of many paper products. The brownstock is typically washed to remove cooking liquor and then processed for the production of unbleached grades of paper products or, alternatively, bleached for the production of high grade, high brightness paper products.
Since chromophoric groups on the lignin are principally responsible for color in the pulp, most methods for the bleaching of brownstock require further delignification of the brownstock. For example, the brownstock may be reacted with elemental chlorine in an acidic medium or with hypochlorite in an alkaline solution to effect this further delignification. These steps are typically followed by reactions with chlorine dioxide to produce a fully bleached product. Oxygen delignification, because it uses inexpensive bleach chemicals and produces by-products which can be burned in a recovery boiler reducing environmental pollutants, is a method that has been used at an increasing rate in recent years for the bleaching of wood pulp in general and brownstock in particular. Oxygen bleaching is frequently followed by subsequent bleach stages of the chlorine or chlorine dioxide type which then require less bleach chemical and produce less environmental pollutants because of the bleaching achieved in the oxygen stage. Unfortunately, most bleaching processes based upon lignin removal have a tendency to degrade a proportion of the holocellulose in the pulp. This degradation is highly undesirable since it adversely affects pulp strength.
In some bleaching processes, the pulp is bleached while being maintained at low to medium levels of pulp consistency. Pulp consistency is a measure of the percentage of solid fibrous material in pulp. Pulps having a consistency of less than about 10% by weight are said to be in the low to medium range of pulp consistency. Processes which require bleaching at low to medium consistency pulp are described in the following patents and publications: U.S. Pat. No. 4,198,266, issued to Kirk et al; U.S. Pat. No. 4,431,480, issued to Markham et al; U.S. Pat. No. 4,220,498, issued to Prough; and an article by Kirk et al. entitled "Low-consistency oxygen delignification in a pipeline reactor--A pilot study", TAPPI, May 1978. Each of the foregoing describe an oxygen delignification step that operates upon pulps in the low to medium consistency range.
Oxygen bleaching of wood pulp is typically carried out on fluffed, high consistency pulp in a pressurized reactor. The consistency of the pulp is typically maintained between about 20% and 30% by weight during the oxygen delignification step. Gaseous oxygen at pressures of from about 80 to about 100 psig is introduced into and reacted with the high consistency pulp. See, G. A. Smook, Handbook for Pulp and Paper Technologists, Chapter 11.4 (1982). In previous oxygen bleaching operations, the pulp from the cooking vessel is dewatered to produce a high consistency mat. The pulp mat is then covered with a thin film or layer of an alkaline solution, generally by spraying the solution onto the surface of the mat. The alkaline solution is typically applied at a rate of about 1.9 to about 7% by weight of dry pulp.
Previously used high consistency oxygen bleaching processes have several disadvantages. In particular, it has now been found that spraying an alkaline solution onto a mat of high consistency pulp does not provide an even distribution of solution throughout the fibrous mass, notwithstanding the generally porous nature of such mats. As a result of this uneven distribution, certain areas of the high consistency mat, usually the outer portions, are exposed to excessive amounts of the alkaline solution. This excessive exposure is believed to cause nonselective degradation of the holocellulosic materials resulting in a relatively weak pulp at least locally. On the other hand, other portions of the high consistency mat, typically the inner portions, may not be sufficiently exposed to the alkaline solution to achieve the desired degree of delignification. Thus, overall quality declines. Applicants have now discovered methods for the production of bleached pulp through high consistency oxygen delignification processes having greater strength and lower lignin content than has been attainable in accordance with prior methods.